Why can't I create url in a nested call attempt - ios

Question

I'm new to Swift and Xcode, and have started porting a small app from Android.
It's not to big so making nested api-calls (like a chain) has worked.
Now I try doing the same in XCode but get error: Failed to construct URL.
I will put the code for the first function that works:
func apiTest(){
/*Setting up for HTTP requests to https://example.com */
let session = URLSession.shared
let url = URL(string: APIBaseUrl+"get")!
let task = session.dataTask(with: url) { data, response, error in
if error != nil || data == nil {
print("Client error!")
return
}
guard let response = response as? HTTPURLResponse, (200...299).contains(response.statusCode) else {
print("Server error!")
return
}
guard let mime = response.mimeType, mime == "application/json" else {
print("Wrong MIME type!")
return
}
do {
let json = try JSONSerialization.jsonObject(with: data!, options: [])
print("OK?")
print(json)
let dict = json as! [String:Any]
print(dict)
print(dict["message"] ?? "Could not read response")
let expected:[String:Any] = ["message": "success"]
print(NSDictionary(dictionary: dict).isEqual(to: expected))
if(NSDictionary(dictionary: dict).isEqual(to: expected)){
//it's working
self.apiTestOk = true
/*Here: continue to a similar function*/
self.apiGetUserPrivateInfo(_id: self.id, _key: self.key)
}
//let message = json as! SimpleMessage //fel i runtime
//print (message.message) //fel i runtime
//let decoder = JSONDecoder()
//let message = try decoder.decode(SimpleMessage.self)
} catch {
print("JSON error: \(error.localizedDescription)")
}
}
task.resume()
/*end http requests*/
}
So I call a similar function with
self.apiGetUserPrivateInfo(_id: self.id, _key: self.key)
That function start like this:
func apiGetUserPrivateInfo(_id: Int, _key: String){
//"get/userp/{id}/{key}"
let session = URLSession.shared
let u:String = APIBaseUrl+"get/userp/\(_id)/\(_key)"
print (u)
var components = URLComponents()
components.scheme = "https"
components.host = "rapport.se/api"
components.path = u
guard let url = components.url else {
preconditionFailure("Failed to construct URL") // here it fails
}
I wonder if it could be the "session" that cant be reused. Would be grateful for an answer.
I also used:
let url = URL(string: u)!
with same result.

Answer 1

The problem is not the session (and, by the way, you generally do want to “reuse” sessions, to avoid overhead), but rather how you’re building URLs, specifically how you’re using the URLComponents.
If you're manually specifying the path for URLComponents, it needs to start with the / character. But URL has methods for building a URL with a path:
guard let baseURL = URL(string: "https://rapport.se/api") else { ... }
let url = baseURL.appendingPathComponent(u)
That’s simpler and more robust way to build up a URL than URLComponents. By the way, this appendingPathComponent is generally a preferred way of composing a URL in general, e.g., instead of:
let url = URL(string: APIBaseUrl+"get")!
You might do:
let url = URL(string: APIBaseUrl)!.appendingPathComponent("get")
This gets you out of the world of worrying about “gee, did my APIBaseUrl end in a / or not”, especially if there’s any risk of some future programmer changing the APIBaseUrl in such a way that the trailing / is removed, suddenly breaking your code. Using URL methods like appendingPathComponent is a robust way of doing this.
If you're wondering when you would use URLComponents, it is most useful if you already have a URL with its path, but just need to add query items to a URL. For example, space characters or & need to be percent escaped when included in a URL, and URLComponents does that for us. Consider:
guard var components = URLComponents(string: "https://example.com") else { ... }
components.queryItems = [
URLQueryItem(name: "q", value: "War & Peace")
]
guard let url = components.url else { ... }
That would result in a URL where the query components would be percent escaped (i.e. the space replaced with %20 and the & replaced with %26):
https://example.com?q=War%20%26%20Peace